The present invention relates to a digital communication network, and more particularly to an integrated circuit used for a high efficiency encoding system such as adaptive differential pulse code modulation.
Recently, due to remarkable developments in digital communication networks, it is possible for various types of analog information to be converted into digital form and transferred among subscribers. A communication system using a predictive encoding system such as adaptive differential pulse code modulation (ADPCM) is widely employed at present. The ADPCM system is advantageous in 1) that speech signals in a telephone network, for example, can be compressed to approximately 16 to 32K-bit/s, which is advantageous considering signal transmission, and 2) that the reproduced signal on the receiving side is high quality. Advances in semiconductor device manufacturing technology with higher integration, such as LSIs, further facilitates wider application of ADPCM systems.
Recent digital communication systems have included transcoder units for performing data compression/expansion. A transcoder unit contains an encoding unit which is provided in the transmitting side for converting a PCM signal into a compressed digital signal, and a decoding system which is provided in the receiving side for decoding the compressed digital signal. By the way, the PCM technique has been widely used in digital communication systems. Particularly, a .mu.-low PCM system or A-low PCM system according to the CCITT recommendation is most popular. When signal processing units, such as the encoding and decoding units, are packed in an LSI chip, it is necessary that the integrated circuit be designed so as to adapt to the above PCM system.
Although the encoding and decoding circuits have many common components, these circuits have been integrated separately and an exclusive encoder LSI and decoder LSI are manufactured independently. Therefore, in the prior art, two types of LSIs are required for the encoder and decoder units. According to an improved example, a circuit arrangement is so designed as to include all the components necessary for the both encoding and decoding circuits and is integrated in an LSI chip. This LSI thus may serve as either an encoder or a decoder. When assembled in the communication system, the LSI is preset so as to perform a selected function, that is, either an encoding or decoding function. In this case, since only one type of LSI can be used as the encoder and decoder units in the communication system, it is possible to reduce the types of LSIs to be manufactured for the encoder and decoder. However, the preset LSI assembled in the system can perform only a selected function, and the function thereof cannot be changed during the system operation. For this reason, the same number of LSIs as the total number of encoder and decoder units, as needed in a digital communication system, must be prepared. Particularly, in the case of a 1:n signal transmission system, such as a telephone conference system wherein one party receives a plurality of speech signals of other parties who are participating in the same conference or meeting, an increased number of LSIs must be used according to the number of signal transmission channels. This causes the configuration of the hardware of the transcoder units included in the digital communication system to be undesirably complicated.